ABSTRACT Anti-cancer therapy has typically been targeted on neoplastic cells. Inhibitors of the enzyme thymidylate synthase (TS), particularly fluoropyrimidines, have been used for many years in the clinical management of a variety of cancers. In spite of the extensive research on the genetic and molecular factors governing tumor response to TS inhibitors, its clinical efficacy remains limited. In this project, we propose a novel approach to increase tumor response to these agents. Tumors are infiltrated with a heterogeneous population of non-neoplastic cells. These include host-derived cells such as fibroblasts, macrophages, lymphocytes, endothelial cells, etc. Together with extracellular matrix, make up the tumor stroma or microenvironment. By secreting an array of cytokines, growth factors, hormones, etc., they play a critical role in tumor growth and progression, as well as response to therapeutic agents. In this proposal, we will test the hypothesis that tumor response to TS inhibitors is governed by the chemosensitivity of infiltrating stromal cells. We will utilize the ApcMin/+ mouse which is predisposed to the development of adenomatous tumors of the small intestine and the colon. By bone marrow transplantation we will generate chimeric mice wherein the chemosensitivity of stromal cells is distinct from that of the tumor. We predict that tumors in these mice will show a drug response that reflects the chemosensitivity of stromal cells. Based on preliminary results, Aim 1 will determine the impact of TS inhibitors on cells in the stromal compartment to identify stromal mediators of response to TS inhibitors. Aim 2, will examine the effect of TS down regulation in stromal cells on tumor response to TS inhibitors. In Aim 3, we will direct sensitization to TS inhibitors specifically to tumor associated stromal cells. In all, the hypothesis being tested in this project will pave the way for development of new treatment modalities using stromal cells to improve therapies targeted at tumor cells to ensure drug induced cancer cell death.